An optical disc represented by a DVD can record a large amount of information signals in high density, so that its use is promoted in many fields of audio, video, and computer. Particularly in recent years, to meet requirements of further increasing in recording capacity, an optical disc having many layers of information recording surfaces has been developed and is already commercially available.
On the other hand, in an optical disc having multilayered information recording surfaces whose distance are small, when a light flux for recording and reproducing information is converged on a certain recording and reproducing surface, the reflected light from the recording and reproducing surface is affected by the reflected light from the neighboring recording and reproducing surface. It causes a fear that the reflected light from the recording and reproducing surface may be recognized as noise. With regard to the problems, there is known an optical pickup apparatus for suppressing noise by combining two wave plates each including two areas having different polarization characteristics as disclosed in the following document:
Sixth Optical Disc Informal Gathering Program Lecture Material “Inter-layer Separation Detection of Multilayer Disc Using Photonic Crystal”, by Tetsuya Ogata, Mar. 17, 2006.
Here, in the optical pickup apparatus disclosed in the above document, a light flux reflected from the information recording surface of the optical disc passes through the first wave plate, then is converged between the first wave plate and the second wave plate, furthermore passes the second wave plate, and passes the polarized beam splitter. When the light flux passes the polarized beam slitter, the noise component is removed and only a normal signal enters the photodetector. However, in the optical pickup apparatus disclosed in the above document, the magnification of the light-converging lens and the interval between the first wave plate and the second wave plate are not specified.
In the optical pickup apparatus, depending on the magnification, it has possibility for example that the light including the normal information and the light including the noise component are hard to be separated and the alignment of the first and second optical elements becomes difficult because the interval between the first wave plate and the second wave plate and an effective area of the first wave plate and the second wave plate becomes small, or alternatively that the size of the optical pickup apparatus becomes excessively large.